Debian 10, Buster Release (2019-07-07). It is pre-built image from BBB web site. I did exactly that way. Give me sometime, as I said I will install it from fresh, so it will make you easier to spot the problem.
The Bug report you posted doesn’t include a resolution.
I found a work-around in my previous thread to mv out the configuration file ( couldn’t remember exact wording).
I am not sure that has no “side-effect”.
The script you posted ( postinst.in) , I don’t know how run it during installation.
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OpenJDK Server VM warning: Option AssumeMP was deprecated in version 10.0 and will likely be removed in a future release.